Hi,

Here the lastest update on Gravity Probe B:

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GRAVITY PROBE B MISSION UPDATE -- June 11, 2004
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Please Note: During the Initialization & Orbit Checkout (IOC)
Phase
of the GP-B mission, we will update our Web site and send out
this
email update once a week (usually on Thursday or Friday) to
keep you
apprised of our progress. From time to time, we may send out
extra
updates, as warranted by mission events.

On its 52nd day in orbit, the spacecraft continues to be in good
health, with all subsystems performing very well. The
spacecraft's
orbit, which will remain in full sunlight through August, is
stable
and meets our requirements for transition into the science
phase of
the mission. All four gyros are digitally suspended and have
passed
several very slow-speed calibration tests. Furthermore, the
science
telescope is locked onto the guide star, IM Pegasi, and we have
verified that it is locked onto the correct star.

Over the past two weeks, through a combination of software
modifications, revised procedures, and commands sent directly
to the
spacecraft, considerable progress has been made in adjusting the
Attitude and Translation Control system (ATC) to properly
maintain
the spacecraft's attitude (pitch, yaw, and roll) in orbit. The
ATC
system accomplishes this important job by controlling the flow of
helium gas, continually venting from the Dewar, through the
spacecraft's micro thrusters. This system is critical to the
success
of the mission because it maintains the required roll rate of the
spacecraft, it keeps the spacecraft and science telescope
pointed at
the guide star, and it keeps the spacecraft in a drag-free orbit.
Thus, the team is particularly gratified to now have the ATC
functioning reliably, with the science telescope locked onto IM
Pegasi.

The "Pegasi" part of the guide star's name indicates that is
located
in the constellation Pegasus; the "IM" prefix (as opposed to a
Greek
letter prefix) indicates that it is a variable star; in fact,
it is
actually part of a binary star system (one of a pair of stars
that
closely orbit each other). On a star map, its location
coordinates
are:

Right Ascension--22 hours 53 minutes 2.27 seconds
Declination--16 hours 50 minutes and 28.3 seconds.

IM Peg is about 300 light years from Earth, and its maximum
magnitude
is 5.85--barely visible to the naked eye. In the Northern
Hemisphere,
you can view the constellation Pegasus in the evening sky from
late
August (rises on the Eastern horizon) to early January (sets on
the
Western horizon).

The process of locking the science telescope onto IM Pegasi
started
with star trackers on either side of the spacecraft locating
familiar
patterns of stars. Feedback from the star trackers was used to
adjust
the spacecraft's attitude so that it was pointing to within a few
degrees of the guide star. The telescope's shutter was then
opened,
and a series of increasingly accurate "dwell scans" was
performed to
home in on the star. Since the spacecraft is rotating along the
axis
of the telescope, imbalance in the rotation axis can cause the
guide
star to move in and out of the telescope's field of view.
Feedback
from the telescope was sent to the ATC system, which adjusted the
spacecraft's attitude until the guide star remained focused in
the
telescope throughout multiple spacecraft roll cycles. The ATC was
then commanded to "lock" onto the guide star.

Finally, to verify that the telescope was locked onto the correct
guide star, the micro thrusters were used to point the
spacecraft/telescope at a known neighboring star, HD 216635 (SAO
1008242), 1.0047 degrees above IM Pegasi. When the telescope was
pointed at this location, the neighboring star appeared with
anticipated brightness, and there were no other stars in the
immediate vicinity. Thus, the sighting of the star, HD 216635,
confirmed the correct relationship between the locations of the
two
stars, ensuring that the telescope is indeed locked onto the
correct
guide star. In addition, the telescope has also seen the star
HR Peg
(HR 8714), a brighter and redder star, located less than half a
degree to the left of IM Pegasi.

This past week the team continued performing calibration tests
of all
gyros, spinning at less than 1 Hz (60 rpm). In addition, the team
successfully tested a back-up drag-free mode of the spacecraft
with
three of the gyros for an entire orbit, and, more
significantly, the
team completed its first successful test of the primary drag-free
mode since re-configuring the micro thrusters, using gyro #3.

In primary drag-free mode, the Gyro Suspension System (GSS) is
turned
off on one of the gyros, so that no forces are applied to it.
The ATC
uses feedback from the position of this gyro in its housing to
"steer" the spacecraft, keeping the gyro centered. Back-up
drag-free
mode is similar, but in this case the GSS applies very light
forces
on the gyro to keep it suspended and centered in its housing.
The ATC
uses feedback from the GSS to "steer" the spacecraft so that
the GSS
forces are nullified or canceled, thereby keeping the gyro
centered.
Applying forces with the GSS to suspend the drag-free gyro adds a
very small, but acceptable, amount of noise to the gyro signal,
and
thus, either primary or back-up drag-free mode can be employed
during
the science experiment. Upcoming milestones include maintaining
the
spacecraft in a drag-free orbit, and beginning gyro calibration
tests
at spin rates of up to 5 Hz (300 rpm).

The spacecraft is being controlled from the Gravity Probe B
Mission
Operations Center, located here at Stanford University. The
Stanford-NASA/MSFC-Lockheed Martin operations team is
continuing to
perform superbly.

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Regards,

LelandJ